[RFC PATCH] bpf: Remove in_atomic() from bpf_link_put().

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: bpf@vger.kernel.org
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	John Fastabend <john.fastabend@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: [RFC PATCH] bpf: Remove in_atomic() from bpf_link_put().
Date: Tue, 9 May 2023 15:24:33 +0200	[thread overview]
Message-ID: <20230509132433.2FSY_6t7@linutronix.de> (raw)

bpf_free_inode() is invoked as a RCU callback. Usually RCU callbacks are
invoked within softirq context. By setting rcutree.use_softirq=0 boot
option the RCU callbacks will be invoked in a per-CPU kthread with
bottom halves disabled which implies a RCU read section.

On PREEMPT_RT the context remains fully preemptible. The RCU read
section however does not allow schedule() invocation. The latter happens
in mutex_lock() performed by bpf_trampoline_unlink_prog() originated
from bpf_link_put().

Remove the context checks and use the workqueue unconditionally.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
The warning can be observed as:
| BUG: sleeping function called from invalid context at kernel/locking/rtmutex_api.c:510
| in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 47, name: rcuc/3
| preempt_count: 0, expected: 0
| RCU nest depth: 2, expected: 0
| CPU: 3 PID: 47 Comm: rcuc/3 Tainted: G            E      v6.3-rt12 #1
| Hardware name: Supermicro X9SCL/X9SCM/X9SCL/X9SCM, BIOS 2.3a 01/06/2021
| Call Trace:
|  <TASK>
|  dump_stack_lvl+0x43/0x60
|  __might_resched+0x137/0x190
|  mutex_lock+0x1a/0x50
|  bpf_trampoline_unlink_prog+0x1b/0x100
|  bpf_tracing_link_release+0x12/0x40
|  bpf_link_free+0x70/0x90
|  bpf_free_inode+0x3e/0x80
|  rcu_core+0x4ff/0x7c0
|  rcu_cpu_kthread+0xa9/0x2f0
|  smpboot_thread_fn+0x141/0x2c0
|  kthread+0x110/0x130
|  ret_from_fork+0x2c/0x50
|  </TASK>

 kernel/bpf/syscall.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 14f39c1e573ee..0adaa1bfbb0d2 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2785,12 +2785,8 @@ void bpf_link_put(struct bpf_link *link)
 	if (!atomic64_dec_and_test(&link->refcnt))
 		return;
 
-	if (in_atomic()) {
-		INIT_WORK(&link->work, bpf_link_put_deferred);
-		schedule_work(&link->work);
-	} else {
-		bpf_link_free(link);
-	}
+	INIT_WORK(&link->work, bpf_link_put_deferred);
+	schedule_work(&link->work);
 }
 EXPORT_SYMBOL(bpf_link_put);
 
-- 
2.40.1

